1. Field of the Invention
The present invention relates to a photometry technique of measuring the optical characteristics of an object in a digital camera, video camera, silver halide camera, or the like.
2. Description of the Related Art
Conventionally, a quick return mirror is inserted into the optical path of a photographing lens for TTL type focus detection. The quick return mirror comprises a main mirror having a semi-transmitting portion, and a sub-mirror positioned behind it. The focus is detected using a light beam which has passed through the main mirror and is deflected by the sub-mirror. In photography after focus detection, the quick return mirror is retracted from the optical path of the photographing lens.
In this focus detection, an actual image sensing plane is different from a focus detection plane, so a focus detection error readily occurs under the influence of a manufacturing error, object characteristics, and the like. For example, if the spectral characteristics of detected light differ between the image sensing plane and the focus detection plane, a focus detection error may occur depending on the spectral characteristics of the object.
This is because the aberrations of a photographing lens are corrected in the visible light region but not corrected for infrared light. Hence, a focus detection error arises from the difference in spectral characteristics between the image sensing plane and the focus detection plane in the infrared region. The image sensing plane of a general camera has spectral characteristics of receiving only visible light. To cope with this, the focus detection plane needs to have the same spectral characteristics as those of the image sensing plane by arranging an infrared cut filter in front of a light receiving unit.
Most cameras detect a focus by projecting a predetermined pattern to an object by auxiliary light and detecting the pattern, in order to detect the focus at low luminance. At this time, the auxiliary light source is an infrared light source using a dominant wavelength of around 700 nm close to visible light so that the object does not feel that auxiliary light is too bright. If the focus detection plane has the same spectral characteristics as those of the image sensing plane, as described above, it cannot receive infrared auxiliary light. Thus, the spectral range of the focus detection plane must be widened by the infrared light source range from that of the image sensing plane. However, a focus detection error occurs if the spectral characteristics differ between the image sensing plane and the focus detection plane.
To solve this, there has conventionally been known a technique of storing the correction amounts of the image sensing plane and focus detection plane in advance, and correcting a focus detection result. However, this correction assumes an object having normal spectral characteristics. A focus detection error occurs for an object under a light source such as a fluorescent light having unique spectral characteristics.
From this, Japanese Patent Publication No. 1-45883 discloses a light receiving apparatus which detects the spectral state of an object and corrects a focus detection result.
FIG. 12 is a sectional view showing the arrangement of a conventional light receiving apparatus. FIG. 13 is a perspective view showing the light receiving apparatus.
The image of an object 21 is formed on a light receiving apparatus 23 via a photographing lens 22. A quick return mirror (not shown) is interposed between the photographing lens 22 and the light receiving apparatus 23. The quick return mirror distributes a light beam having passed through the photographing lens 22 into light beams to the image sensing plane (not shown) and the light receiving apparatus 23.
The light receiving apparatus 23 comprises an optical path split prism 26 having a semi-transmitting surface 24 and total reflection surface 25, and first and second light receiving element arrays 28 and 29 formed on a board 27. With this arrangement, the image of the same portion of the object 21 is formed on the light receiving element arrays 28 and 29 to detect the focus of the photographing lens 22 by a known phase difference detection method.
Light beams having all wavelengths can enter the first and second light receiving element arrays 28 and 29. First and second auxiliary light receiving elements 30 and 31 are arranged on the board 27, as shown in FIG. 13. The auxiliary light receiving elements 30 and 31 respectively support an infrared cut filter 32 which transmits visible light (400 to 650 nm) and cuts infrared light, and an infrared transmitting filter 33 which transmits near infrared light (700 to 800 nm) and cuts visible light. The auxiliary light receiving elements 30 and 31 separately output signals representing the quantities of visible light and near infrared light contained in a light beam from the photographing lens 22.
Focus detection results obtained by the light receiving element arrays 28 and 29 are corrected based on the ratio of signals detected by the auxiliary light receiving elements 30 and 31. This enables focus detection under all light sources.
However, this prior art suffers the following problems.
Since the light receiving element arrays 28 and 29 and the auxiliary light receiving elements 30 and 31 are arranged on the focus detection board 27, this unit that different positions of an object are measured. When correcting a detected focus, the accurate spectral state of a position of the object cannot be obtained. The light receiving ranges of an object by the auxiliary light receiving elements 30 and 31 are very small and influenced by a partial object.
When many focus detection regions are set at high density in a wide photographing range, as disclosed in Japanese Patent No. 3363683, many light receiving element arrays 28 and 29 are formed at high density on the focus detection board 27. There is no space to arrange the auxiliary light receiving elements 30 and 31.
Further, when a camera has a TTL viewfinder, like a single-lens reflex camera, light guided to the focus detection system is dark because it has passed through the half-mirror. Since the auxiliary light receiving elements 30 and 31 are very small, it is difficult to detect the spectral state of an object at low luminance.